General consensus suggests these ruins to the rear of the Hall are the remains of a previous building, Lower Scout and that John Mitchell built Scout Hall to adjoin to this. There does not appear to be an adjoining door, but it strikes me that the two buildings were connected as there remains a low lying wall between the two.
For now, there ruin is as enigmatic as the Hall itself, and I suspect will be such for a while longer.
It is only really visible now most of the flora has died off, in the summer it was overrun with Himalayan Balsam and the internal room & doorway were completely concealed.
Some recent photos: